1. Field of the Invention
This invention relates to a process for producing a toner for developing electrostatic latent images to form toner images in image-forming processes such as electrophotography and electrostatic printing, or a toner for forming toner images in an image-forming process of a toner jet system; and a toner obtained by this production process. More particularly, this invention relates to a process for producing a toner used in a fixing system in which these toner images are fixed to transfer mediums such as printing sheets by the action of heat and pressure.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are conventionally known as methods for electrophotography. In general, recorded images are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium such as paper as occasion calls, followed by fixing by the action of heat, pressure, heat-and-pressure, or solvent vapor.
In the foregoing, as methods for developing electrostatic images by the use of toners or methods for fixing toner images to paper or the like, a variety of methods have ever been proposed, and methods suited for the intended image forming processes are employed. Then, toners used for such purpose have commonly been produced by pulverization processes in which colorants comprising dyes and/or pigments are melt-mixed and uniformly dispersed or dissolved in thermoplastic resins to form resin-colorant dispersions, and thereafter such molten products are cooled, followed by pulverization and classification by means of a fine grinding mill and a classifier, respectively, to produce toners having the desired particle diameters.
Reasonably good toners can be produced by such a process conventionally carried out for producing toners by pulverization, but there is a certain limit, i.e., a limit to the range in which toner materials are selected. For example, the above resin-colorant dispersions must be brittle enough to be pulverizable with ease by means of economically available production apparatus. However, such resin-colorant dispersions made brittle in order to meet these requirements tend to result in a broad range of particle diameter (particle size distribution) of the toner particles formed when actually pulverized at a high speed, especially causing a problem that fine particles are included in a relatively large proportion. Moreover, there is a problem that toners obtained from such highly brittle materials tend to be further finely pulverized or powdered when used for development in copying machines or the like.
In this method, it is also difficult to perfectly uniformly disperse solid fine particles of colorants or the like in the resin, and some toners may cause an increase in fog, a decrease in image density and a lowering of color mixing properties or transparency, depending on the degree of dispersion. Accordingly, care must be well taken when colorants are dispersed. Also, colorants and other internal additives may come bare to rupture sections of toner particles to cause fluctuations in developing performance.
In order to overcome the problems of the toners produced by such pulverization and also meet requirements for higher image quality, higher minuteness and lower energy consumption, toners produced by polymerization are energetically on researches. For example, Japanese Patent Publications No. 36-10231, No. 43-10799 and No. 51-14895 disclose methods of producing toners by suspension polymerization. Japanese Patent Applications Laid-open No. 60-220358 and No. 63-205665 also disclose methods of producing toners by emulsion polymerization; Japanese Patent Application Laid-open No. 61-273553, a method of producing a toner by dispersion polymerization; and Japanese Patent Application Laid-open No. 60-258203, a method of producing a toner by seed polymerization in which a monomer is absorbed in seed particles and the monomer is polymerized inside the seed particles.
Since these methods have no step of pulverization at all, colorants and other internal additives do not come bare to the surfaces of toner particles and hence the toner particles can have a uniform triboelectric charging performance. These methods have such an advantage. Also, since these methods make it possible to omit the step of classification, these are greatly effective for cost reduction on account of energy saving, reduction of production time, improvements in process yield and so forth.
In particular, the seed polymerization can make toners have higher function, e.g., in respect of the colorants and other internal additives coming bare to toner particle surfaces, enables additional formation of one or more binder resin layers on toner particle surfaces, or enables formation of a core/shell structure in which, e.g., a low-softening substance is encapsulated by changing the polarity of a monomer to be added, or enables surface modification of toner particles by using a compound with low surface energy such as fluorine resin. Thus, this is a technique desired to be further advanced.
In the above seed polymerization, researches have ever been put forward aiming principally at how a polymerizable monomer be fed to seed particles. For example, as disclosed in Japanese Patent Applications Laid-open No. 61-215602, No. 62-121701, No 64-1702 and No. 05-232741, in such a method a polymerizable monomer composition containing at least a polymerizable monomer and a polymerization initiator is first dispersed in the form of oil droplets in water containing a surface-active agent, and then the resultant dispersion is added to an aqueous dispersion of seed particles. As a consequence, the polymerizable monomer and the polymerization initiator dissolve out of the oil droplets into the water in a very small quantity, so that the polymerizable monomer and the polymerization initiator are absorbed into the seed particles, and finally the polymerization takes place in the interiors of the seed particles. In this method, the polymerizable monomer may be added in a quantity of approximately from 0.01-fold by weight to 1,000-fold by weight based on the weight of the seed particles. Hence, this method has applicability over a wide range, and has an advantage that it can employ formulation suited for various designs.
However, studies made by the present inventors have revealed that the above seed polymerization produces fine powder secondarily in a large quantity at the same time when the seed particles are enlarged, and the presence of such fine powder causes a lowering of performances of toner especially in an environment of high temperature and high humidity. Such fine powder is considered to be secondarily produced because emulsion polymerization or suspension polymerization takes place concurrently in the reaction system at its part other than the seed particles. More specifically, a surface-active agent is necessary in order to disperse the seed particles and the polymerizable monomer composition (oil droplets) in water, where the surface-active agent having been used to disperse the oil droplets come to remain in the system as the polymerizable monomer and so forth are absorbed from the oil droplets in the seed particles. Because of such an excess surface-active agent, the polymerizable monomer and polymerization initiator having dissolved out of the oil droplets into water in a very small quantity are not completely absorbed in the seed particles to make the emulsion polymerization or suspension polymerization take place concurrently in the reaction system to form the fine powder, as so considered.
For the purpose of solving the above problem, a method in which the seed polymerization is carried out in the presence of a water-soluble polymerization inhibitor is proposed as disclosed in Japanese Patent Application Laid-open No. 3-237105. This is a method in which only the seed polymerization is made to proceed in the interiors of seed particles so that any fine powder due to emulsion polymerization taking place concurrently in the water which is a dispersion medium can be kept from occurring to keep the whole fine powder less occur. Such a method can lessen the secondary production of the fine powder due to the emulsion polymerization taking place concurrently. However, according to studies made by the present inventors, this method has found not to be effective against the secondary production of the fine powder due to the suspension polymerization.
Meanwhile, as disclosed in Japanese Patent Application Laid-open No. 11-218960, a toner production method is proposed in which a water-soluble polymerization initiator is used when a polymerizable monomer capable of forming a polymer having a higher glass transition point than the glass transition point the seed particles have is polymerized to produce a toner having core-shell structure. However, according to studies made by the present inventors, the polymerizable monomer added in order to form the shell has found to undergo emulsion polymerization to form particles by itself and not to form any shells on the seed particle surfaces, because in such a method a surface-active agent and the water-soluble polymerization initiator are each used in a large quantity.
Accordingly, it has been sought to provide, in a toner production process in which shells are formed on core particles by seed polymerization, a process of producing a toner by seed polymerization with good efficiency, which can uniform the surface state and coverage of shells formed on individual seed particles and may less secondarily produce any impurities such as fine powder.
An object of the present invention is to provide a toner production process having solved the problems the above related background art has had. More specifically, an object of the present invention is to provide, in a toner production process having the step of seed polymerization to polymerize a polymerizable monomer in a dispersion medium in which base particles stand dispersed, a process of producing a toner by seed polymerization with good efficiency, which can control any scattering of coverage which may be caused between toner particles and besides may less secondarily produce the fine powder.
Another object of the present invention is to provide a toner having superior properties which is obtained by the above production process.
The above object can be achieved by the present invention described below. That is, the present invention is a process for producing a toner, having the step of polymerizing a polymerizable monomer in the presence of base particles containing at least a binder resin, to enlarge and/or surface-modify the base particles, wherein a polymerization initiator having in one molecule a hydrophilic moiety and a hydrophobic moiety and a reactive moiety between them is used in the above polymerization.
The present invention is also a toner comprising toner particles containing at least a binder resin, which toner particles have a circularity within the range of from 0.92 to 1.0;
the toner particles being toner particles obtained by polymerizing a polymerizable monomer in the presence of base particles containing at least a binder resin, using a polymerization initiator having in one molecule a hydrophilic moiety and a hydrophobic moiety and a reactive moiety between them, to enlarge and/or surface-modify the base particles.